A centering mechanism and process for condenser lenses in field emission scanning electron microscopes

Abstract

The present application relates to the technical field of field emission scanning electron microscope, in particular to a centering mechanism and process for condenser in field emission scanning electron microscope, comprising a centering mechanism and a condenser lens barrel, characterized in that: the condenser lens barrel is designed as an independent module in an electron optical column, the condenser lens barrel is composed of a condenser support barrel, a condenser and a titanium tube, the condenser is installed in the condenser support barrel, the condenser is fixed relative to the condenser support barrel in the Z-axis direction, and in the XY direction, the condenser can be moved in the condenser support barrel through the condenser centering mechanism, so that the center of the main electron beam passes through the magnetic field center of the condenser. By changing the supporting mode of the condenser, the centering of the condenser is realized, after centering, the condenser is very firm, and almost does not drift with the residual stress of related parts.

Description

Technical Field

[0001] This invention relates to the field of field emission scanning electron microscopy, and more particularly to a centering mechanism and process for a condenser lens in a field emission scanning electron microscope. Background Technology

[0002] Field emission scanning electron microscopes (SEMs) typically contain two condenser lenses (magnetic lenses): the first condenser lens, in conjunction with the objective aperture, is used to adjust the electron beam current, and the second condenser lens is used to adjust the half-angle of the electron beam when it is finally focused onto the sample (related to diffraction). Some SEMs also include a third condenser lens, which, in conjunction with the second condenser lens, adjusts the half-angle of the electron beam and, simultaneously, adjusts the angle of the signal electrons.

[0003] All condenser lenses must be precisely aligned with the main electron beam (the deviation should be less than 5 micrometers, and even less than 1 micrometer for high-end electron microscopes). Otherwise, not only will aberrations occur, but the electron beam will also deflect. The magnitude of the electron beam deflection angle is directly proportional to the strength of the focusing static magnetic field and the distance between the electron beam and the center of the magnetic field. If the electron beam is not precisely aligned with the magnetic field of the condenser lens, all subsequent mechanical and electromagnetic alignment settings will need to be readjusted once the condenser lens is adjusted; if the electron beam deflection angle is too large, the electron beam may be extinguished.

[0004] The electron beam from the field source may fail to pass through the magnetic field center of the condenser lens for various reasons (the deviation may be as high as 0.5 mm): (1) The flatness tolerance of the field source support causes the electron beam to be at an angle to the axis when it starts, and the distance from the axis is magnified when it reaches the condenser lens; (2) The extraction pole and the accelerating pole are not completely perpendicular to the axis as a whole; (3) The preceding axis alignment device causes the electron beam to be off-axis in order to align a certain lens; (4) The processing error of the condenser lens and the microscopic defects of the material cause the magnetic field to be inherently misaligned with the axis, etc.

[0005] Condensers are generally aligned mechanically, with the condenser tube designed as a double-layer structure: the outer layer is the support tube, and the inner layer is the condenser. The condenser is aligned with the electron beam by moving relative to the support tube in the X and Y directions through alignment settings. The alignment of the electron beam is determined by wobbling. The previous alignment method used set screws: the condenser was fixed to the bottom of the support tube using a smooth, flexible material (usually PTFE pads), so that the vertical position of the condenser could not be changed. The X and Y directions were adjusted by two opposing set screws. This method has the following problems: (1) When one set screw pushes the condenser, the other set screws must be relaxed, and the set screws can only push the condenser forward. Alignment in one direction requires the gradual approach of two opposing set screws, which makes the alignment process of the condenser very difficult and time-consuming. Repeatedly pushing, wobbling, and relaxing each set screw can take several days to align a condenser. (2) It mainly relies on the elastic force generated by the compression of the flexible PTFE pad to press and keep it fixed. The PTFE pad is particularly prone to fatigue. After a period of time, the pressing force will decrease, the rigidity will deteriorate, the natural frequency will decrease, and the focusing lens will easily resonate with external vibrations. (3) The four set screws press against the cylindrical surface. The contact area is very small, and the stress on the contact surface is large. It is easy to loosen due to deformation, which will cause the focusing lens to drift. Summary of the Invention

[0006] The purpose of this invention is to solve the problems in the background art by proposing a centering mechanism for a condenser lens in a field emission scanning electron microscope.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a centering mechanism for a condenser lens in a field emission scanning electron microscope, comprising a centering mechanism and a condenser lens barrel. The condenser lens barrel is designed as an independent module in the electron optics column. The condenser lens barrel consists of a condenser lens support cylinder, a condenser lens, and a titanium tube. The condenser lens is installed inside the condenser lens support cylinder. In the Z-axis direction, the condenser lens is fixed relative to the condenser lens support cylinder. In the XY direction, the condenser lens can be moved within the condenser lens support cylinder by the condenser lens centering mechanism so that the center of the main electron beam passes through the magnetic field center of the condenser lens.

[0008] Preferably, the condenser lens is cylindrical, and four support blocks of the same size are provided on the cylindrical surface of the condenser lens. The support surfaces of the four support blocks are all located on a plane perpendicular to the cylinder axis. The support surface and the upper bottom surface of the support block are rectangular surfaces with the arc removed, and the other surfaces are rectangular. The four support blocks are evenly distributed on the cylindrical surface, and the radial center lines of two adjacent support blocks are angled at 90°.

[0009] Preferably, the condenser lens support cylinder is cylindrical, and four identical annular steps are arranged on the inner wall of the support cylinder from top to bottom. The platform of the four steps is on a plane perpendicular to the axis, and the steps are evenly distributed along the cylindrical surface of the inner wall of the condenser lens support cylinder, that is, the included angle between the radial center planes of two adjacent steps is 90°.

[0010] Preferably, the position of the platform of the step on the condenser lens support tube corresponds to the position of the support surface of the support block on the condenser lens. When the condenser lens is installed in the condenser lens support tube and is in contact with the support surface of the support block on the condenser lens, there is a certain gap between the bottom surface of the condenser lens and the inner bottom surface of the condenser lens support tube. The platform of the step is larger than the support surface of the support block. When the condenser lens is aligned, the support block moves on the platform to ensure that no interference occurs.

[0011] Preferably, a screw hole perpendicular to the outer surface is provided at the center of the outer surface of the support block on the condenser lens. The screw hole is used to connect the screw of the centering mechanism, so the thread and size should be matched.

[0012] Preferably, the condenser lens support cylinder has a radial hole above the platform of the inner wall step. The screw passes through the radial hole and is connected to a support block on the condenser lens. The size of the radial hole is in a suitable position, and the diameter of the radial hole is larger than the diameter of the screw to ensure that the screw does not collide with the hole wall when the condenser lens is aligned.

[0013] Preferably, the components for moving and fixing the condenser in the XY direction are a self-locking nut and a disc spring washer. The disc spring washer can reduce the drift of the condenser after centering caused by stress.

[0014] Furthermore, this invention also discloses an alignment process for a condenser lens in a field emission scanning electron microscope, the specific steps of which are as follows: S1. The mechanism by which the condenser lens moves in the XY direction is the condenser lens clamping mechanism; S2. The condenser lens is pressed tightly by the PTFE pad on the bottom surface of the upper disk of the titanium tube. S3. The support surface of the condenser lens support block is kept in close contact with the stepped platform of the inner wall of the condenser lens support cylinder, so that the condenser lens cannot move up and down during centering.

[0015] Compared with existing technologies, the advantages of this invention are as follows: This invention is an improvement on the mechanical alignment mechanism of the condenser lens of a field emission electron microscope. The main improvement is that the set screw is replaced with a screw rod, and four screw rods are connected and secured to the condenser lens through four screw holes. The fastening nut outside the support cylinder uses the outer wall (plane) of the support cylinder as a fulcrum to move the condenser lens, allowing it to move forward or backward, making alignment simpler and faster. After alignment and tightening of the all-metal self-locking nut, the condenser lens is very stable. Although the alignment process requires the use of a flexible PTFE gasket for fixation in the Z direction, after alignment, the Z-direction fixation mainly relies on the frictional force of the metal material for locking, fundamentally improving the rigidity of the structure, and preventing the condenser lens from resonating with external vibrations. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view of the condenser lens barrel in the XZ or YZ plane of the present invention; Figure 2 This is a cross-sectional view of the condenser lens barrel at the center position of the centering mechanism in this invention. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0018] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0019] Reference Figures 1-2 A centering mechanism for a condenser lens in a field emission scanning electron microscope includes a centering mechanism and a condenser lens barrel. The condenser lens barrel is designed as an independent module in the electron optics column. The condenser lens barrel consists of a condenser lens support tube, a condenser lens, and a titanium tube. The condenser lens is installed inside the condenser lens support tube. In the Z-axis direction, the condenser lens is fixed relative to the condenser lens support tube. In the XY direction, the condenser lens can be moved within the condenser lens support tube by the condenser lens centering mechanism so that the center of the main electron beam passes through the magnetic field center of the condenser lens.

[0020] The condenser lens is cylindrical, and four support blocks of the same size are set on the cylindrical surface of the condenser lens. The support surfaces of the four support blocks are all located on a plane perpendicular to the cylinder axis. The support surface and the upper bottom surface of the support block are rectangular surfaces with the arc removed, and the other surfaces are rectangular. The four support blocks are evenly distributed on the cylindrical surface, and the radial center lines of two adjacent support blocks are angled at 90°.

[0021] The condenser lens support cylinder is cylindrical, and there are four identical annular steps on the inner wall of the support cylinder from top to bottom. The platform of the four steps is on a plane perpendicular to the axis. The steps are evenly distributed along the cylindrical surface of the inner wall of the condenser lens support cylinder, that is, the included angle between the radial center planes of two adjacent steps is 90°.

[0022] The position of the platform of the step on the condenser lens support tube corresponds to the position of the support surface of the support block on the condenser lens. When the condenser lens is installed in the condenser lens support tube and is in contact with the support surface of the support block on the condenser lens, there is a certain gap between the bottom surface of the condenser lens and the inner bottom surface of the condenser lens support tube. The platform of the step is larger than the support surface of the support block. When the condenser lens is aligned, the support block moves on the platform to ensure that no interference occurs.

[0023] A screw hole perpendicular to the outer surface is opened at the center of the outer surface of the support block on the condenser lens. The screw hole is used to connect the screw of the centering mechanism, so the thread and size should be matched.

[0024] The condenser lens support cylinder has a radial hole above the platform of the inner wall step. The screw passes through the radial hole and is connected to a support block on the condenser lens. The size of the radial hole is in a suitable position, and the diameter of the radial hole is larger than the diameter of the screw to ensure that the screw does not collide with the hole wall when the condenser lens is aligned.

[0025] The components that move and fix the condenser in the XY direction are the self-locking nut and the disc spring washer. The disc spring washer can reduce the drift of the condenser after centering caused by stress.

[0026] Furthermore, this invention also discloses an alignment process for a condenser lens in a field emission scanning electron microscope, the specific steps of which are as follows: S1. The mechanism by which the condenser lens moves in the XY direction is the condenser lens clamping mechanism; S2. The condenser lens is pressed tightly by the PTFE pad on the bottom surface of the upper disk of the titanium tube. S3. The support surface of the condenser lens support block is kept in close contact with the stepped platform of the inner wall of the condenser lens support cylinder, so that the condenser lens cannot move up and down during centering.

[0027] Figure 1This is a cross-sectional view of the condenser lens barrel in the XZ or YZ plane. 1 is a CF flange (316L) for a sealed connection to the lens barrel above the electron optics column. 2 is a titanium tube (TA2) serving as the vacuum channel for the electron beam. To ensure ultra-high vacuum, titanium tube 2 is integrally formed with CF flange 1 using a dissimilar metal welding process. 3 is the condenser lens support cylinder, also made of TA2, designed to prevent changes in the sealing gap between the lower end of titanium tube 2 and the bottom of support cylinder 3 due to thermal expansion and contraction.

[0028] 4 is the CF flange (316L), which is integrally processed with the support cylinder 3 using a dissimilar metal welding process, and is used for a sealed connection with the lens barrel under the electron optics column. 5 is the PTFE gasket, which uses the elastic force caused by compression to firmly fix the condenser lens to the platform of the condenser lens support step 9. 6 is the condenser lens yoke, generally made of permalloy with high magnetic permeability (actually it should be assembled from two parts, but since it is irrelevant to this invention, it is omitted). 7 is the condenser lens coil. 8 is the sealing ring, preferably a perfluorocarbon seal. 9 is the condenser lens support step; the support platform should be carefully polished to reduce friction during alignment. 10 is the main electron beam. 11 is the magnetic field generated by the condenser lens. 12 is the condenser lens alignment mechanism; the alignment mechanism should be positioned as high as possible where possible to minimize the vertical friction and alignment thrust distance, resulting in a smaller overturning torque. This is the main content of the invention, which will utilize... Figure 2 Detailed explanation.

[0029] Figure 2 This is a cross-sectional view of the condenser lens barrel at the center of the centering mechanism. The central part is irrelevant to this invention and is omitted.

[0030] 1 is the condenser lens support cylinder, and the centering mechanism installation location on the outer surface of the cylinder should be cut into a plane perpendicular to the radial direction.

[0031] 2 is the magnetic yoke of the condenser lens.

[0032] 3. Condenser Support Block: This is part of the condenser yoke, meaning the condenser yoke is machined as a single piece. The outer surface of the condenser support block is rectangular, precisely perpendicular to the line connecting the center of the circle to its center. Its size depends on the fine-pitch screw 4; it is recommended that the length and width of the rectangle be greater than three times the screw diameter. The lower surface of the support block should be carefully polished to reduce friction during alignment. A radial screw hole should be drilled at the center of the outer surface of support block 3 for mounting the screw; a depth of 10 mm is recommended. In the condenser yoke machining process, the screw hole should be drilled before high-temperature annealing, leaving a certain allowance during tapping. After high-temperature annealing, the screw should be finely finished during finishing to prevent residual stress at the screw from causing creep in the focusing lens position and leading to alignment failure. Fine finishing the screw will also introduce new residual stress; it is recommended to perform a low-temperature annealing after fine finishing to relieve stress (harmless to the focusing lens) and make the stress on the screw surface more uniform. This screw hole is crucial for the stable alignment of the condenser lens, so it should be handled carefully; otherwise, alignment may fail after a period of time.

[0033] 4 is a fine-pitch screw (M6 / M8 recommended), made of 316L stainless steel, used for connecting the condenser lens from the outside. Screw 4 must also have very low residual stress, so it cannot be purchased directly from the market and should be custom-made by a specific factory. To ensure low residual stress, the machining process must be: rough machining before high-temperature annealing (leaving allowance), and fine finishing after high-temperature annealing.

[0034] 5 is a butterfly spring pad, which allows for thermal expansion and contraction and magnetostriction of the condenser lens, and must also be processed and customized according to stress relief technology.

[0035] 6 is an all-metal fine-tooth self-locking nut, which can be purchased from the market, but you must buy a well-known brand, as this will increase reliability and reduce the difficulty of centering.

[0036] 7 is the movement clearance, which allows the condenser lens to move within the support tube. The size of the movement clearance 7 depends on the machining and assembly precision of other parts in the electron optics column; 1 mm should be sufficient.

[0037] 8 is a screw through hole, a hole at the installation location of the centering mechanism of the support cylinder wall. The screw will pass through this hole to connect with the condenser lens, and its diameter should be appropriately larger than the screw diameter.

[0038] Assembly steps of the condenser lens barrel: (1) Align the condenser lens roughly and install it into the support tube; (2) Cover the condenser lens with the PTFE gasket; (3) Install the sealing ring and titanium tube and tighten the relevant screws; (4) Pass the four screws through the screw through hole 8 to connect with the condenser lens and tighten them; (5) Use a fine needle to apply epoxy resin glue to the joint between the screw 4 and the support block 3 through the gap between the screw 4 and the screw through hole 8 (do not contaminate the possible vacuum inner wall) to lock the screws. In this way, the condenser lens barrel becomes an independent module in the electron optics column.

[0039] The condenser tube is mounted in the electron optics column, which is then mounted on the test stage (a simple homemade tool for observing the electron beam spot). The relevant power supply is connected, a vacuum is drawn, and the gun is fired (allowing the source to emit the electron beam). These processes are irrelevant to this invention and will not be described in detail. Now, focusing and alignment can be performed. The alignment process is as follows: 1. Install disc spring washers and lock nuts on the two adjacent centering mechanisms (do not install them on the other two).

[0040] 2. Enable the wobbling function. Wobbling is a program that periodically changes the excitation current of the condenser lens at a certain frequency.

[0041] 3. Gently tighten the lock nut by hand and observe the change in the beam pattern. If the beam pattern only changes in size but its position remains unchanged, it indicates that the alignment has been completed.

[0042] 4. Install the other two lock nuts.

[0043] 5. Use a torque wrench to slowly and periodically tighten each lock nut. Ensure that the torque wrench reading is the same or close to the same value for each nut each time. After final tightening, ensure that the tightening force of each nut is the same or close to the same. Due to friction, the tightening force of a single tightening may change later. If the tightening force of each nut is different, thermal expansion and contraction or magnetostriction may cause misalignment.

[0044] 6. Finally, check the beam spot. If the center position of the beam spot remains unchanged, the alignment is complete. Otherwise, repeat the alignment process.

[0045] To further clarify, the aforementioned fixed connection should be interpreted broadly unless otherwise explicitly specified and limited. For example, it may be welding, gluing, or integral molding, or other conventional methods well known to those skilled in the art.

[0046] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A centering mechanism for a condenser lens in a field emission scanning electron microscope, comprising a centering mechanism and a condenser lens barrel, characterized in that: The condenser lens barrel is designed as an independent module in the electron optics column. The condenser lens barrel consists of a condenser lens support tube, a condenser lens, and a titanium tube. The condenser lens is installed inside the condenser lens support tube. In the Z-axis direction, the condenser lens is fixed relative to the condenser lens support tube. In the XY direction, the condenser lens can be moved inside the condenser lens support tube by the condenser lens centering mechanism so that the center of the main electron beam passes through the magnetic field center of the condenser lens.

2. The centering mechanism for a condenser lens in a field emission scanning electron microscope according to claim 1, characterized in that: The condenser lens is cylindrical, and four support blocks of the same size are provided on the cylindrical surface of the condenser lens. The support surfaces of the four support blocks are all located on a plane perpendicular to the cylinder axis. The support surface and the upper bottom surface of the support block are rectangular surfaces with the arc removed, and the other surfaces are rectangular. The four support blocks are evenly distributed on the cylindrical surface, and the radial center lines of two adjacent support blocks are angled at 90°.

3. The centering mechanism for a condenser lens in a field emission scanning electron microscope according to claim 2, characterized in that: The condenser lens support cylinder is cylindrical, and four identical annular steps are arranged on the inner wall of the support cylinder from top to bottom. The platform of the four steps is on a plane perpendicular to the axis. The steps are evenly distributed along the cylindrical surface of the inner wall of the condenser lens support cylinder, that is, the included angle between the radial center planes of two adjacent steps is 90°.

4. The centering mechanism for a condenser lens in a field emission scanning electron microscope according to claim 3, characterized in that: The position of the platform of the step on the condenser lens support tube corresponds to the position of the support surface of the support block on the condenser lens. When the condenser lens is installed in the condenser lens support tube and is in contact with the support surface of the support block on the condenser lens, there is a certain gap between the bottom surface of the condenser lens and the inner bottom surface of the condenser lens support tube. The platform of the step is larger than the support surface of the support block. When the condenser lens is aligned, the support block moves on the platform to ensure that no interference occurs.

5. The centering mechanism for a condenser lens in a field emission scanning electron microscope according to claim 4, characterized in that: A screw hole perpendicular to the outer surface is provided at the center of the outer surface of the support block on the condenser lens. The screw hole is used to connect the screw of the centering mechanism, so the thread and size should be matched.

6. The centering mechanism for a condenser lens in a field emission scanning electron microscope according to claim 5, characterized in that: The condenser lens support cylinder has a radial hole above the platform of the inner wall step. The screw passes through the radial hole and is connected to a support block on the condenser lens. The size of the radial hole is in a suitable position, and the diameter of the radial hole is larger than the diameter of the screw to ensure that the screw does not collide with the hole wall when the condenser lens is aligned.

7. A centering mechanism for a condenser lens in a field emission scanning electron microscope according to claim 6, characterized in that: The components that move and fix the condenser in the XY direction are a self-locking nut and a disc spring washer. The disc spring washer can reduce the drift of the condenser after centering caused by stress.

8. A centering process for a condenser lens in a field emission scanning electron microscope, employing a centering mechanism for a condenser lens in a field emission scanning electron microscope as described in claims 1-7, characterized in that: Includes the following steps: S1. The mechanism by which the condenser lens moves in the XY direction is a condenser lens clamping mechanism; S2. The condenser lens is pressed tightly by the PTFE pad on the bottom surface of the upper disk of the titanium tube; S3. The support surface of the condenser lens support block is kept in close contact with the stepped platform of the inner wall of the condenser lens support cylinder, so that the condenser lens cannot move up and down during centering.

Images